Electrochemical energy storage devices and components

ABSTRACT

A battery electrode composition is provided comprising anode and cathode electrodes and an electrolyte ionically coupling the anode and the cathode. At least one of the electrodes may comprise a plurality of active material particles provided to store and release ions during battery operation. The electrolyte may comprise an aqueous metal-ion electrolyte ionically interconnecting the active material particles. Further, the plurality of active material particles may comprise a conformal, metal-ion permeable coating at the interface between the active material particles and the aqueous metal-ion electrolyte. The conformal, metal-ion permeable coating impedes water decomposition at the aforesaid at least one of the electrodes.

CLAIM OF PRIORITY UNDER 35 U.S.C. § 119

The present application for patent is a Continuation of U.S. patentapplication Ser. No. 14/222,312 entitled “ELECTROCHEMICAL ENERGY STORAGEDEVICES AND COMPONENTS” filed Mar. 21, 2014, which in turn claimspriority to Provisional Application No. 61/804,166 entitled“ELECTROCHEMICAL ENERGY STORAGE DEVICES AND COMPONENTS” filed on Mar.21, 2013, and to Provisional Application No. 61/832,114 entitled“ELECTROCHEMICAL ENERGY STORAGE DEVICES AND COMPONENTS” filed on Jun. 6,2013, which are expressly incorporated by reference herein.

BACKGROUND Field

The present disclosure relates generally to energy storage devices, andmore particularly to metal-ion battery technology and the like.

Background

Among the metal-ion batteries, Li-ion battery technology has achievedthe greatest commercial success, owing to the very high gravimetriccapacity (3860 mAh/g) and moderately high volumetric capacity (2061Ah/L) of Li anodes combined with the high activity of Li and the highmobility of Li ions in various hosts.

Yet, other metal-ion batteries may also offer reasonably high volumetricand gravimetric energy densities. For example, the gravimetric specificcapacity of Al (2980 mAh/g, calculated based on the three-electronoxidation of Al) is close to that of Li, while its volumetric storagecapacity (8043 Ah/L) is four times higher than that of Li, due to thefivefold higher density of Al. The excellent storage capacity of Alcombined with its broad availability (Al is the most abundant metal inthe Earth's crust, contributing to over 8% of the total mass) and lowcost, makes it an attractive anode material. Similarly, Mg, for example,is nearly as abundant as Al, but it is more active than Al and has highgravimetric (2233 mAh/g) and volumetric (3885 Ah/L) specific storagecapacities. Na-ion and Ca-ion batteries may also offer some advantagesin selected applications. Finally, batteries that combine metal cationsand non-metal anions may also be utilized in various applications.

Unfortunately, current Li-ion battery technology utilized fortransportation, grid storage, and electronic device fields is expensive,slow, and unsafe. Such cells utilize organic electrolytes and sufferfrom several limitations. Formation of Li dendrites in commercialbatteries is particularly challenging to detect and prevent. Whenformed, they may lead to internal shorts, which give rise to localheating, melting of the separator, thermal runaway, and eventually fire.The high flammability of organic electrolytes does not help thissituation. In addition, decomposition of organic electrolytes with thepresence of water and other impurities limit the cycle life of Li-ionbatteries and make their assembling expensive. Further, the relativelylow ionic conductivity of organic electrolytes combined with the lowionic conductivity of the solid electrolyte interphase (SEI) limits thepower performance of Li-ion batteries.

The use of aqueous chemistry may significantly improve the safety ofLi-ion battery technologies, and, at the same time, reduce the cost ofLi-ion cells and corresponding battery packs. However, the use ofaqueous electrolytes is known to typically limit the maximum voltage ofaqueous Li-ion and other metal-ion batteries to below around 1.2-1.5V.This low voltage limits the energy density of the cells. In addition,the electrode fabrication and cell construction developed forconventional Li-ion chemistry utilizing organic electrolytes is veryexpensive. Adoption of similar manufacturing technology for aqueousLi-ion cells will increase their manufacturing cost.

Accordingly, there remains a need for improved aqueous metal-ionbatteries, components, and other related materials and manufacturingprocesses.

SUMMARY

Embodiments disclosed herein address the above-stated needs by providingimproved aqueous metal-ion (such as Li-ion) battery components, improvedbatteries made therefrom, and methods of making and using the same. Suchaqueous metal-ion batteries facilitate the incorporation of advancedmaterial synthesis and electrode fabrication technologies, and enablefabrication of high voltage and high capacity aqueous metal-ionbatteries at a cost lower than that of conventional Li-ion batterytechnology.

A battery electrode composition is provided comprising anode and cathodeelectrodes and an electrolyte ionically coupling the anode and thecathode. At least one of the electrodes may comprise a plurality ofactive material particles provided to store and release ions duringbattery operation. The electrolyte may comprise an aqueous metal-ionelectrolyte ionically interconnecting the active material particles.Further, the plurality of active material particles may comprise aconformal, metal-ion permeable coating at the interface between theactive material particles and the aqueous metal-ion electrolyte. Theconformal, metal-ion permeable coating impedes water decomposition atthe aforesaid at least one of the electrodes.

The conformal, metal-ion permeable coating may have an average thicknessis in the range of about 10 nm to about 500 nm. The conformal, metal-ionpermeable coating may encase each of the active material particlesindividually. Alternatively or in addition, the conformal, metal-ionpermeable coating may encase the plurality of active material particlesas a whole. In some designs, the conformal, metal-ion permeable coatingmay be generally uniform, while in other designs it may have anon-uniform composition that changes gradually with radial distance(e.g., from an inner surface to an outer surface). In this case, a morechemically and mechanically robust coating may be formed.

In various embodiments, the conformal, metal-ion permeable coating maybe a composite coating comprising a plurality of layers. For example,the plurality of layers may comprise an outer layer formed from anelectrical insulator material for preventing electrochemical reductionof the aqueous metal-ion electrolyte on the anode or preventingelectrochemical oxidation of the aqueous metal-ion electrolyte on thecathode. This may be achieved by the insulative outer layeraccommodating a portion of the voltage drop between the anode andcathode, thereby reducing the voltage drop across the aqueous metal ionelectrolyte. In other examples, the plurality of layers may comprise anelectrically conductive layer for electrically connecting the activematerial particles, an interfacing layer for enhancing uniformity oradhesion of another layer, a mechanically stable layer for enhancingmechanical stability of the conformal, metal-ion permeable coating, or asupplemental protection layer for preventing electrochemical reductionof the aqueous metal-ion electrolyte on the anode or preventingelectrochemical oxidation of the aqueous metal-ion electrolyte on thecathode.

In some applications, the conformal, metal-ion permeable coating maycomprise, as a single or outer layer, a chemically-linked, polymericcoating containing one or more pH-regulating functional groups. As anexample, the one or more pH-regulating functional groups may comprise anacidic functional group for decreasing the pH (e.g., to pH ofapproximately 4 or below) of active particles at the cathode to preventelectrochemical oxidation of the aqueous metal-ion electrolyte. Asanother example, the one or more pH-regulating functional groups maycomprise a basic functional group for increasing the pH (e.g., to pH ofapproximately 9 or above) of active particles at the anode to preventelectrochemical reduction of the aqueous metal-ion electrolyte. Ineither case, the one or more pH-regulating functional groups may beborne by one or more polymers attached to the surface of the activematerial particles.

In some designs, the conformal, metal-ion permeable coating may beformed on the aforesaid at least one of the electrodes prior to aformation cycle of a cell comprising the battery composition, while inother designs it may be at least partially formed on the aforesaid atleast one of the electrodes by decomposition of one or more additives tothe aqueous metal-ion electrolyte during a formation cycle of a cellcomprising the battery composition. That is, the coating layer(s) can bedeposited on the electrode surface either (1) prior to assembling of thecell or (2) formed in-situ during the so-called formation cycle(s) ofthe cell when additive(s) to an aqueous electrolyte decompose at apotential where water does not yet decompose, thus forming a protectivecoating on the electrode surface, or (3) both.

In different designs, the conformal, metal-ion permeable coating maycomprise (i) a carbon or (ii) one or more metals that enhanceover-potential for water decomposition by at least 0.25 V. Theconformal, metal-ion permeable coating may also comprise a plurality ofpores having an average pore size in the range of about 0.5 nm to about10 nm.

The active material particles may be composites with a core-shellstructure. The core of each active material particle may be, forexample, a nanocomposite comprising active material and at least one of(i) pores, (ii) a carbon additive, or (iii) a carbon scaffolding matrix.The carbon scaffolding matrix may be porous with an average pore size inthe range of about 0.5 nm to about 20 nm, and may contain activematerial that at least partially fills the pores. The shell of eachactive material particle may be, for example, a nanocomposite comprisingat least one of (i) an electrical conductive layer, (ii) a mechanicalstability layer, or (iii) a water impermeability layer.

In general, the metal-ion battery may correspond to an aqueous Li-ionbattery, or other such aqueous metal-ion batteries.

Various methods of fabricating a battery electrode compositioncomprising active particles are also provided. They may comprise, forexample: providing active material particles to store and release ionsduring battery operation; electrically connecting the active particleswith a current collector; forming a conformal protective coating on theelectrode surface in such a way that the electrode remains porous whileall (or at least a substantial portion) of its open pore surface area iscovered with such a coating. For connecting the active particle togetherduring the electrode fabrication, the method may involve mixing theactive particles with a binder and annealing at an elevated temperatureto cause solidification of the bonded particles in a particular shape.In some embodiments, the surface of the active particles may allowsintering particles together at elevated temperatures and thus notrequire a binder.

In some embodiments, the shape of the produced electrodes may be planar(for sandwich-type electrode stacking with a separator layer in betweenpositive and negative electrodes). In other embodiments, the shape ofthe produced electrodes may be cylindrical (for cylindrical cellfabrication with a cylindrical electrode in another hollow cylinderelectrode and a cylindrical separator layer in between cylindricalpositive and negative electrodes).

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are presented to aid in the description ofembodiments of the invention and are provided solely for illustration ofthe embodiments and not limitation thereof.

FIG. 1 illustrates a stability profile for water (H₂O) across pH.

FIG. 2 illustrates an electrochemical cell design for localizing pH atthe electrodes to enhance the aqueous electrolyte stability voltagerange.

FIG. 3 provides examples of various pH shifting and chemical bondinggroups that may be used in conjunction with the cell design of FIG. 2.

FIG. 4 is a schematic view of polymer chain adsorption of a pH-modifyingcoating on an electrode substrate surface.

FIG. 5 provides two graphs illustrating the impact of pH-regulatingcoatings on the electrochemical stability of a pH-neutral aqueouselectrolyte.

FIG. 6 is a cross-sectional view of an electrode illustrating the use ofan electrically insulative but ionicially conductive conformal coating.

FIG. 7 illustrates the voltage drop between the anode and the cathode ofan aqueous cell with and without a protective coating of the type shownin FIG. 6.

FIGS. 8-10 are schematic illustrations of different examples of in-situformation of the protective coating layer on an electrode via differentsuitable precursors.

FIG. 11 illustrates an example multi-layer implementation of theprotective coating layer impeding aqueous electrolyte decomposition.

FIG. 12 is a cross-section view of different example particle designsincorporating one or more Li-ion permeable, but solvent impermeableprotective shell(s).

FIG. 13 provides an example of a high capacity aqueous Li-ion batterywith a pH-modified anode and cathode.

FIG. 14 provides an example of different porous particle designscontaining a conversion-type active material (sulfur) that experiencesvolume changes upon Li insertion.

FIG. 15 is a flow chart illustrating an example method of fabricating abattery electrode composition comprising active particles.

FIG. 16 shows a comparison of two cell constructions, including aconventional Li-ion cell side by side an aqueous Li-ion cell asdescribed herein.

FIG. 17 shows select performance characteristics of the two cellconstructions, including a conventional Li-ion cell side by side anaqueous Li-ion cell as described herein.

DETAILED DESCRIPTION

Aspects of the present invention are disclosed in the followingdescription and related drawings directed to specific embodiments of theinvention. The term “embodiments of the invention” does not require thatall embodiments of the invention include the discussed feature,advantage, process, or mode of operation, and alternate embodiments maybe devised without departing from the scope of the invention.Additionally, well-known elements of the invention may not be describedin detail or may be omitted so as not to obscure other, more relevantdetails.

Aqueous metal-ion (such as Li-ion) technology may offer enhanced safety,enhanced power performance and reduced cost compared to traditionalLi-ion technology that utilizes organic electrolyte(s). Organicelectrolytes used in conventional Li-ion batteries exhibit specificLi-ion conductance of up to about 3 mS/cm. In contrast, Li ions inaqueous solutions exhibit conductance of about 75 mS/cm. Thus, for thesame electrodes and current rate, organic electrolytes may induce abouta twenty-five times higher polarization. Therefore, Li-ion battery cellswith aqueous electrolyte(s) may operate at more than an order ofmagnitude higher current densities and accordingly provide an order ofmagnitude higher power. Conversely, for the same power performance,aqueous Li-ion batteries may utilize thicker electrodes.

The key bottlenecks in the development of stable, low-cost, aqueousLi-ion technology, however, include: (i) a low thermodynamically stablevoltage range for aqueous electrolytes; (ii) the absence of stableelectrode materials that offer high capacity; and (iii) high cost andpoor compatibility of traditional Li-ion cell manufacturing techniqueswith aqueous Li-ion technologies.

The improvements in aqueous Li-ion battery technology described hereinaddress the above-noted challenges, and may be implemented via one ormore of several complimentary techniques, including but not limited to:(1) different techniques for increasing the voltage stability range ofpH-neutral aqueous electrolytes by forming ion-permeable coatings on theelectrode surface that impede aqueous electrolyte decomposition as wellas the resulting gas generation and self-discharge; (2) differenttechniques for reducing the cost of electrode fabrication and aqueouscell assembling; and (3) different techniques for forming advancednanostructured high-capacity electrodes compatible with aqueouschemistry.

In the description below, several examples are provided in the contextof aqueous Li-ion batteries because of the current prevalence andpopularity of Li-ion technology. However, it will be appreciated thatsuch examples are provided merely to aid in the understanding andillustration of the underlying techniques, and that these techniques maybe similarly applied to various other metal-ion batteries, such asaqueous Na-ion, aqueous Ca-ion, aqueous K-ion, aqueous Mg-ion, and otheraqueous metal-ion batteries.

In addition, various aspects of the present disclosure may be applied tovarious aqueous electrochemical capacitors, aqueous pseudocapacitors,aqueous Li-ion capacitors, aqueous asymmetric supercapacitors, hybridelectrochemical capacitor-battery devices (where one of the electrodesis battery-like, while the other is electrochemical capacitor-like), andother aqueous electrochemical energy storage devices in order to enhancetheir performance (for example, to enhance maximum charge voltage or toreduce leakage current, or both). Further, various aspects of thepresent disclosure may also be applied to electrochemical energy storagedevices based on non-aqueous electrolytes.

According to different embodiments, various aspects of the presentdisclosure may be applied to both the positive electrode and thenegative electrode of aqueous electrochemical energy storage devices, orto the electrodes individually (either the positive electrode or thenegative electrode). Application to only one of the electrodes may beused to prevent aqueous electrolyte decomposition on such an electrode.For example, application to a cathode in particular may help preventoxygen evolution at higher potentials. Application to an anode inparticular may help prevent hydrogen evolution at lower potentials.

Several methods are described below to enhance the aqueous electrolytestability voltage range. For example, in a first method, a pHmodification of the electrode surface may be implemented. This may beparticularly beneficial for pH-neutral aqueous electrolytes. In a secondmethod, a conformal coating may be formed on the electrode surface toaccount for some of the voltage drop between the electrodes, allowingliquid electrolyte to be maintained within a stable potential range.This may be generally applied to electrolytes other than pH-neutralaqueous electrolytes.

FIG. 1 illustrates a stability profile for water (H₂O) across pH. Asshown, at high potentials, H₂O decomposes with O₂ evolution, and at lowpotentials, with H₂ evolution. The potential of H₂O oxidation at thecathode, 2H₂O→O₂(g)+4H⁺+2e⁻, is governed by the Nernst equation and canbe increased to above 1.2 V (vs. NHE) at low pH values. Similarly, thepotential of H₂O reduction at the anode, 2H⁺+2e⁻→H₂(g) orH₂O+2e⁻→H₂(g)+2OH⁻, can be reduced to below −1 V (vs. NHE) at high pHvalues.

FIG. 2 illustrates an electrochemical cell design for localizing pH atthe electrodes to enhance the aqueous electrolyte stability voltagerange. In this design, the surfaces of both electrodes, including ananode 202 and a cathode 204, are functionalized with pH-tuning moieties203, 205 of macromolecules without changing the pH in the bulk of apH-neutral aqueous Li-ion electrolyte solution 206 (such as solutions ofLi₂SO₄, LiCl, LiNO₃, or other Li salts in H₂O).

This design is advantageous in that the local pH value can beindependently adjusted at the surface of each electrode via thepH-tuning moieties confined to the surfaces of corresponding electrodes.Polymers or macromolecules bearing these functional moieties have beenfound to be particularly useful for this purpose. Such macromoleculescan be physically or chemically attached to the surface of the electrodematerial, affecting the pH value only locally, without changing the pHin the bulk of the battery electrolyte solution (as is furtherillustrated in corresponding average pH distribution shown in FIG. 2).

FIG. 3 provides examples of various pH shifting and chemical bondinggroups that may be used in conjunction with the cell design of FIG. 2.The decrease of pH in the vicinity of the electrode can be achieved byattaching polymer-bearing acidic groups, such as carboxylic, phosphoric,or sulfuric moieties. For simplicity, such polymers may be referred toas “acidic.” Depending on the pKa of the acidic group in the polymer,the local pH value can be tuned in wide ranges from about pH=6 to aboutpH=0. Among the above-mentioned acids, sulfuric acid is the strongest(with pKa less than 1), thus providing the largest local pH shift. Inorder to shift the pH near the battery electrodes into basic conditions,polymers bearing amine moieties in their structure can be used. Forsimplicity, such polymers may be referred to as “basic.” Depending onthe pKa of the amine used, the local pH values can be varied from aboutpH=7 to about pH=12. Another way to increase the local pH is to bind aweak acid polyanion (polymer containing weak acidic groups) salt of astrong base. In the case of an aqueous Li-ion battery, the choice of astrong base is predefined to be LiOH. Due to hydrolysis, salts formedfrom a strong base and a weak acid will increase pH locally.

Long-term stability of the pH-modifying coatings may be enhanced bychemical bonding to the particle surface and/or coating cross-linking(e.g., via the chemical bonding groups shown in FIG. 3). To obtain a pHmodifying polymer capable of chemically bonding to the electrodesurface, two monomers may be co-polymerized. One co-monomer may bear apH-modifying group. The second co-monomer may contain in its structure achemical group capable of forming covalent bonds with particles ofactive materials. By changing the ratio between the two co-monomers, thebonding and pH tuning properties of the polymer coating can be tuned formore optimized electrode performance.

FIG. 4 is a schematic view of polymer chain adsorption of a pH-modifyingcoating on an electrode substrate surface. The polymeric chemistry ofthe pH-modifying coating will provide long-term stability. When polymersare adsorbed, they form multiple contacts with the surface 402 called“trains” 404, as shown. “Loop” sections 406 and “tail” sections 408 arenot connected to the substrate. On average, the train fraction forrelatively high molecular weight polymers adsorbed on the surface may beabout 0.15-0.25 and 3-4 monomeric units may be involved in each trainsection. This means that a polymer chain with a typical degree ofpolymerization N=1,000 has at least (0.15)*(3)*(1000)=450 contacts withthe surface. Despite each contact being a relatively weak bond, thelarge number of the contacts results in very strong interaction betweenthe surface and the polymer molecule, often exceeding the strength ofcovalent bonds.

The pH-tuning polymers may be utilized as an additional surface coatingon the surface of metal-ion battery electrodes or as binders used in thepreparation of battery electrodes. Epoxy groups have been found to beparticularly suitable for permanent chemical attachments to varioussurfaces. In particular, they can react with metal oxides to formchemical bonds between metal oxide and pH-modifying polymers. Similarly,epoxy groups are capable of binding with functionalities intrinsicallypresent on carbon surfaces, such as carboxyl groups. In order to inducecross-linking of the polymer coatings, carbon-carbon double bonds withinthe polymer structure can be utilized.

In some applications, it may be important for the produced coatings toremain permeable to electrolyte solvent (such as water in the case ofaqueous metal-ion batteries). The pH-modifying units located in the“loops” and “tail” sections of the attached polymer coating are notlinked to the surface. These groups are polar, and, therefore, areeasily hydrated by water molecules providing both the required pH shiftand, equally important, channels for easy active ion migration in andout of the active electrode material. From the ratio of “trains” to“loops” (e.g., 0.15-0.25 to 0.85-0.75) in the polymer macromoleculecoatings, it can be estimated that about 75-85% of the particle surfacewill have free access for active ions. Therefore, formation of thepolymer coatings may have a very minor effect on the powercharacteristics of the aqueous metal-ion batteries.

FIG. 5 provides two graphs illustrating the impact of pH-regulatingcoatings, on the surface of glassy carbon working electrodes, on theelectrochemical stability of a pH-neutral aqueous electrolyte (1M LiCl)measured in a 3-electrode configuration. On the left, it can be seenthat the voltage stability range is expanded to below −1.2 V vs. NHE bycoating a carbon surface with a polymer bearing basic moieties. On theright, it can be seen that the voltage stability range is expanded toover 1.5 V vs. NHE by coating a carbon surface with a polymer bearingacidic functional moieties. The higher current observed for the carbonsurface with a polymer bearing acidic functional moieties is likelyrelated to the pseudo-capacitance induced by the acidic functionalgroups of the polymer coating.

It will be appreciated that pH-modifying coatings may be depositeddirectly on the surface of active particles or on the surface of anotherlayer that coats the active particles and may additionally serve variousother functions, such as, for example, additionally prevent waterdecomposition on the electrode surface, prevent degradation of activematerial, improve electrical conductivity within the electrode, orimprove the interface between the active particles and the pH-modifyingcoating, to name a few.

As discussed above, it will be appreciated that pH-modifying coatingsmay be used for other chemistries of anodes and cathodes as well as forelectrochemical capacitor applications and hybrid devices.

In some applications, in order to further minimize H₂ evolution, anodesmay additionally be provisioned with microporous and mesoporousadditives, capable of proton and H₃O⁺ adsorption, and known to preventwater decomposition at low potentials. Such additives may be provided inthe form of a coating around the active particles (or electrode) or inthe form of individual particles, or even in the form of electrolyteadditives.

In some configurations, electrolyte additives may be used to createoxide/hydroxide coatings with a basic nature deposited on top of theelectrodes. For example, in the case of metal nitrates as additives tothe battery electrolyte, metal oxide/hydroxide coatings can be formedduring electro-reduction at the electrode. Ions of, for example, Mg²⁺,Al³, Cr³⁺, Fe³⁺, Mn³⁺, and Co²⁺, can be reduced during the process.However, this approach may not be applicable to nitrates of metals suchas Cu, Tl, Bi, and Pb, and yields only metal deposits. Utilization ofperchlorate salts of Cu, Tl, Bi, and Pb results in hydroxide/oxide filmformation during electro-reduction.

Another method for synthesizing oxide films is metal-ion galvanostaticreduction in the presence of hydrogen peroxide. Coatings consisting ofZrO₂, Al₂O₃, Al₂O₃—ZrO₂, and Al₂O₃—Cr₂O₃ can be made by this approach.

Oxide coatings on the battery electrode can be obtained, for example, bya two-step process. In the first step, a metal coating may be depositedon the electrode by electroplating. In the second step, the metalcoating may be converted into an oxide by electro-oxidation. Oxides ofthe metal, which can be electrodeposited from aqueous solutions, can bedeposited in this way.

The desired coating porosity and enhanced proton adsorption can beachieved by gentle heat treatment in the case of hydroxide coatings.Heat treatment leads to partial dehydration of the coating, creatingporosity. A deposition regime (e.g., galvanostatic, potentiostatic,current pulsing, or voltage pulsing) can also be utilized for thecreation of microporosity in the coatings.

In some applications, porous metal (or porous carbon) coatings or porousmetal (or porous carbon) powder may efficiently prevent H₂ evolution onthe anode or O₂ evolution on the cathode. Several metals are known tooffer high over-potentials for H₂ and O₂ evolution, and have been foundto be useful as additives for aqueous Li-ion batteries. For example,iron (Fe) increases the potential of O₂ generation at the cathode byabout 0.75 V, nickel (Ni) by 0.56 V, lead (Pb) by 0.81 V, and graphiteby 0.95V. Other metals, for example zinc, bismuth, and mercury, alsosignificantly increase the potential of water decomposition at thecathode. All these materials can be used as coatings or as a powder incathode construction. Similarly, several metals decrease the increase ofH₂ generation at the anode. For example, graphite, lead, zinc, mercury,and bismuth lower the potential of water decomposition and H₂ evolutionon the anode by at least 0.6 V. All these materials can be used ascoatings or as a powder in cathode construction. In some configurations,the presence of micropores and mesopores within such materials has beenfound to further prevent water decomposition.

In some configurations (for example, when it is advantageous to reducethe cost of the electrode fabrication or to increase the electrodedensity), the coating of conductive carbon or selective metal(s) maypreferably be not porous.

FIG. 6 is a cross-sectional view of an electrode illustrating the use ofan electrically insulative but ionicially conductive conformal coating.In this example, a thin protective coating 602 is provided to cover theelectrode surface via active particles 604 electrically connected to acurrent collector 606. In some applications, it may be advantageous toform such a conformal, electrically insulative (i.e., essentially orsubstantially impermeable to electrons) but ionically conductive (i.e.,essentially or substantially permeable to ions participating in energystorage) conformal coatings on the surface of electrodes for aqueousmeal-ion batteries.

Conventionally, the voltage between the anode and the cathode of anaqueous cell is applied across an aqueous electrolyte layer. When such avoltage exceeds some critical value (often in the range of about 0.6 Vto about 1.9 V) water decomposition takes place with oxygen evolution onthe cathode or hydrogen evolution on the anode, or both. However, if oneor both electrodes are coated with a thin electrically insulative butionically conductive protective layer, this voltage drops across boththe electrolyte and the protective layer in series. This provides aparticular advantage for stabilizing an aqueous electrolyte againstdecomposition.

FIG. 7 illustrates the voltage drop between the anode and the cathode ofan aqueous cell with and without a protective coating of the type shownin FIG. 6. As shown, if, for example, the total ionic (e.g., Li ion)resistance of this protective layer(s) approximately equals the ionicresistance of the aqueous electrolyte, the voltage drop across theaqueous electrolyte becomes approximately half of the potentialdifference between the anode and the cathode. If, for example, by usingpH modifying moieties on the surface of the protective layer, thestability range of an aqueous electrolyte can approach 1.9 V, then themaximum voltage between the anode and the cathode may safely approach3.8 V because half of that voltage will be dropped across the protectivelayer. In this case, the voltage of such an aqueous Li-ion cell, forexample, approaches that of the conventional Li-ion cell with an organicelectrolyte. This high voltage increases the energy density of theaqueous Li-ion cell, which is particularly important for practicalapplications.

According to various embodiments, the overall ionic resistance of theprotective layer(s) can be adjusted to provide an optimum combination ofhigh total cell voltage, power performance, and reliability. Further,the protective layer may be applied to an anode, a cathode, or both. Ifapplied to an anode, it may prevent hydrogen evolution at low anodepotentials. If applied to a cathode, it may prevent oxygen evolution athigh cathode potentials.

In many applications, it may be advantageous for this protective layerto uniformly coat the electrolyte-accessible surface of the (porous)electrode. This is because non-uniformities in the layer thickness mayinduce undesirable variations in the resistivity of the protectivelayer. If some portion of the protective layer becomes too thin in somearea of the electrode, the voltage drop across the aqueous electrolytemay exceed a critical value leading to water decomposition. If someportion of the protective layer becomes too thick in some area of theelectrode, it will impede the ion transport in this area, limitingcapacity utilization at high current densities. For practical reasons,it may be desirable to have no more than a three-fold variation in thethickness of the protective layer within the protected electrode.

In some applications, it may be advantageous for the overall coatingthickness of the protective coating layer to range from about 10 nm toabout 500 nm. Thinner coatings may be prone to defects. In some cases,coatings thinner than 5 nm may allow quantum mechanical tunneling of theelectrons, which is undesirable as it will permit electrochemicalreduction or oxidation of water at extreme potentials and may preventthe protective coating from function properly. Coatings thicker than 500nm may impede ion transport or contribute to a significant portion ofthe total mass or volume, which may also be undesirable.

The ionic conductivity of the protective layer may be made relativelylow. For example, when the effective diffusion distance of Li ions inthe aqueous electrolyte is 1.6 mm, its ionic resistance (per 1 cm² areaof the electrode) will be equal to (0.16 cm)*(1/0.075 mS cm⁻¹)=2.1 Ohm,assuming ionic conductance of the aqueous Li electrolyte to be 75 mS/cm.By way of example, consider a design in which the porous electrodesurface area is 100 times larger than the geometrical area of theelectrode (due to internal porosity) and that this surface is uniformlycoated with the protective layer. In this example, the thickness of theprotective layer is 20 nm and its resistance is set to 2.1 Ohm.Accordingly, the Li ionic conductance of this layer will be a mere(0.000002 cm)/(100)/(2.1 Ohm)≈10⁻⁸ S cm⁻¹. When the effective diffusiondistance of Li ions in the aqueous electrolyte is larger (e.g., 8 mm forexample), the Li ionic conductance of this layer must be even smaller, amere ≈10⁻⁹ S cm⁻¹. This is a relatively low value, and easy to achievein many water-compatible ceramic and polymer materials. It does notrequire development of water-compatible highly conductive solidelectrolytes.

The application of such conformal protective coating(s) on the porouselectrode surface provides several key advantages over, for example, athick solid conductive membrane layer that separates the aqueouselectrolyte from a solid nonporous electrode or a porous electrodefilled with a non-aqueous electrolyte. First, the conformal protectivecoatings do not require high conductance for providing high overallpower performance. Second, in most cases, these coatings aresignificantly less expensive to deposit because their thicknesses arequite small and because they do not need to possess high ionicconductance. Third, such coatings are more resistant to failure becauseeven if one particle fails (e.g., due to a coating defect) and reactswith the electrolyte, the whole cell can continue to function, losingonly a tiny fraction of the overall capacity. Furthermore, as discussedelsewhere herein, the defect may be sealed or repaired during cycling byusing additives within the electrolyte. In contrast, the highconductivity thick membranes (typically 10-500 microns) that may, inprinciple, also be used, suffer from high prices that make themuncompetitive and low conductivity that fail to provide high powerperformance. More importantly, if a large defect develops within such amembrane, it may ruin the entire cell because the individual particlesare not protected.

Formation of the insulative but ionically conductive protective layerconformal coatings on the electrode surface can be performed viaelectro-reduction (on the anode) or electro-oxidation (on the cathode)of ceramic precursors dissolved in aqueous electrolyte. For example,electro-reduction of the metal ions on the anode can be used tosynthesize a variety of metal hydroxide or oxide films. The oxideformation instead of Me electro-deposition can be achieved by bathcomposition. For example, metal nitrates will yield hydroxide (oxide)films. Examples include, but are not limited to, ions of Mg²⁺, Al³⁺,Cr³⁺, Fe³⁺, Mn³⁺, and CO₂+. However, salts of Cu, Tl, Bi, and Pb yieldonly metal deposits in the case of nitrate counter ions. Utilization ofperchlorate salts of Cu, Tl, Bi, or Pb results in hydroxide (oxide)formation during electro-reduction.

Another method for synthesizing oxide films is galvanostatic reductionin the presence of hydrogen peroxide. Coatings consisting of ZrO₂,Al₂O₃, Al₂O₃—ZrO₂, and Al₂O₃—Cr₂O₃ can be made by this approach.

Oxide coatings on the battery electrode can be obtained, for example, bya two-step process. In the first step, a metal coating is made byelectroplating. In the second step, the metal coating is converted intooxide by electro-oxidation. Oxides of the metal, which can beelectrodeposited from aqueous solutions, can be deposited in this way.

Metal oxide/hydroxide films can be generated by oxidation at thecathode. The pH of the electrolyte is chosen in such a way that thelower oxidation state is stable while the higher oxidation state readilyundergoes hydrolysis to yield the metal oxide or hydroxide. Examplesinclude, but are not limited to, MnO₂, PbO₂, V₂O₅, MnO(OH), and CoO(OH).

By fine-tuning the applied cell potentials, the oxidizing or reducingpower can be continuously varied and suitably selected. Galvanostatic,potentiostatic, and cyclic voltammetry (CV) modes of deposition or theircombinations can be utilized for formation of the coating with desiredproperties.

Formation of the insulative but ionically conductive protective layerconformal coatings on the electrode surface may also be performed viaelectro-grafting of monomers present in an electrolyte solution. In thiscase, it is preferable that electro-grafting takes place at potentialswhere the majority of electrolyte solvent remains stable. In someapplications, it may be preferable for the electro-grafting to takeplace in-situ during the first cycle of the aqueous metal-ion battery.In this case, a monomer should be dissolved in this electrolyte aqueoussolution. In some applications, this electro-grafting may be employed asa secondary safety measure; that is, if the pre-deposited coating failsin some part of the electrode or in some part of an active particle dueto a manufacturing defect, this water decomposition site will beneutralized by in-situ formation of the grafted layer.

In one example, a vinyl monomer present in the electrolyte solution maybe used as a precursor for electro-grafting. Upon battery charging, anegative potential applied to an anode will cause reduction of thedouble bond of the vinyl monomer, causing anion formation, which, inturn, will cause monomer polymerization and grafting to the electricallyconductive electrode (electron conductive) or electrically conductivesite(s) on the electrode surface.

FIGS. 8-10 are schematic illustrations of different examples of in-situformation of the protective coating layer on an electrode via differentsuitable precursors.

In one example, acrylonitrile may be electro-grafted on the electrodesurface, as shown schematically in FIG. 8. Via proper design of the(meth)acrylate monomers, electro-grafting in water media is also anoption, as shown schematically in FIG. 9. Three major structuralfeatures of the monomer have been found to be advantageous in thisregard: (i) a long hydrophobic alkyl chain capable of expelling waterfrom the electrical double layer of the battery electrode and increasingthe electrochemical window of the aqueous electrolyte; (ii) the cappingof this chain by a cationic hydrophilic head at one end in order totrigger micellization and desorption to the anode surface; and (iii) thecapping of the second chain-end by a polymerizable acrylic fragment.

Other examples of a suitable precursor for the in-situ formation of theprotective coating layer on an electrode (such as the anode) arediazonium salts' derivatives. These molecules can be cleaved whenelectro-reduced on the battery anode, as shown schematically in FIG. 10.The radicals formed as a result of an electron transfer from theconductive anode surface (or conductive site on the anode surface)eventually induce formation of a covalent bond with the electrode.Because the electro-grafted molecules are neutral, no polyadditionreaction occurs (in contrast to the electro-reduction of acrylicmonomers). The nature of the substituent R in the aromatic ring can betuned in order to achieve the desired ionic resistance of the coatinglayer.

Careful selection of the electro-grafting conditions (such as reagentconcentration, grafting potential, and, when grafting is performed in adifferent cell, pH of the grafting solution) allows for a stable surfacelayer formation with a desired morphology and precise control of filmthickness and ionic resistivity.

FIG. 11 illustrates an example multi-layer implementation of theprotective coating layer impeding aqueous electrolyte decomposition. Inthis example, the multilayer coating structure includes one or moreinner layers 1102, one or more intermediate layers 1104, and one or moreouter layers 1106 disposed on or around active particles 1108, althoughit will be appreciated that the number and arrangement of the differentlayers may vary from application to application as desired. Each of thelayers may bear different functions.

An inner layer may be deposited, for example, to assist in electricallyconnecting active particles of the electrode. In this case, this layershould be made electrically conductive. Examples of materials for such alayer include but are not limited to a conductive carbon coating or aconductive metal coating, which should be stable in the potential rangefor the electrode of interest. Nickel is an example of such a metal thatis suitable for some anodes.

An intermediate layer can also be deposited in order to assist informing uniform coating of any subsequent layers. Examples of materialsfor such a layer include but are not limited to metal(s), metalalloy(s), metal oxide(s), metal fluoride(s), metal sulfide(s), variousother ceramic coatings, polymer(s), and composite(s), to name a few. Itis desirable that this material should also be stable in the potentialrange for the electrode of interest and not undergo undesirable phasetransformation reactions.

Another intermediate layer can also be deposited in order to enhance themechanical properties of the overall coating or enhance mechanicalstability of individual particles. Examples of materials for such alayer include but are not limited to carbon, metal(s), metal alloy(s),metal oxide(s), metal fluoride(s), various other ceramic coating(s), andcomposite(s), to name a few.

One or more outer layer(s) may be deposited to provide additionalprotection against aqueous electrolyte decomposition or other usefulfunctions. Examples of materials for such a layer include but are notlimited to various metal(s) (as previously described), metal oxide(s),metal fluoride(s), metal sulfide(s), various other ceramic coatings,polymer(s) and composite(s), to name a few. It is desirable that thismaterial should also be stable in the potential range for the electrodeof interest and not undergo undesirable phase transformation reactions.

All layers should be permeable to ion transport in order to provideenergy storage capability to the active particles. In some applications,it may be preferred that at least one of the layers does not allowelectron transport, thus preventing electrochemical reduction of theaqueous electrolyte on the anode or preventing electrochemical oxidationof the aqueous electrolyte on the cathode. In this case, an electricalinsulator of sufficient thickness (e.g., typically greater than about 5nm) should be used to prevent electron tunneling. This function shouldalso be maintained during cycling without forming electron conductionpaths by, for example, phase transformation or defect formation.

In some applications, it may be advantageous for the most outer layer tocontain pH-regulating moieties that change the local pH in the vicinityof the electrode, thus assisting in preventing aqueous electrolytedecomposition, as described in more detail above.

In some applications, it may be beneficial for some of the coatinglayer(s) to be deposited on the electrode surface prior to assembling ofthe cell. In this case, high flexibility can be achieved in both thechemistry and morphology of the layer(s). In some applications, it maybe beneficial for at least the outer coating layer(s) to be formedin-situ during the so-called formation cycle(s) of the cell whenadditive(s) to an aqueous electrolyte decompose at a potential, wherewater does not yet decompose, thus forming a protective coating on theelectrode surface. In this case, the overall cost of the cellfabrication can be reduced. In some applications (for example, whenmultiple protection mechanisms are desired), the coating layer(s) may bedeposited both prior to cell assembling and during cycling. Thedecomposition of electrolyte additives may also provide a protectionagainst defects formed during electrode handling or during celloperation. Such defects ordinarily allow local undesirable waterdecomposition in some portion of the electrode, leading toself-discharge, gas generation, and cell degradation. The decompositionof the electrolyte additives may “heal” such defects and allow long-termcycle stability to be achieved.

The coating layer(s) on the electrode surface may be deposited by one ormore vapor deposition technique(s), electroless deposition,electrodeposition, dip coating, sol-gel, or other known methods ofconformal deposition of coatings.

In some applications, an overall coating thickness (not counting thepH-modifying moieties, if present) in the range of about 5 nm to about500 nm may be advantageous. Thinner coating may be prone to defects.Thicker coatings may impede ion transport or contribute to a significantportion of the total mass or volume, which is undesirable.

In some applications, it may be advantageous for the protective coatingto gradually change in composition. In this case, the internal stressesduring cycling may be reduced and delamination of the coating prevented.

In some applications, it may be advantageous for the protective coatingto contain micropores or mesopores. The presence of such pores mayenhance the stability range of aqueous electrolytes. In addition, suchpores may accommodate some of the volume changes within the activematerial particles, thus stabilizing the mechanical integrity of theelectrode during cycling.

Many intercalation-type active materials are compatible with aqueousLi-ion batteries. Examples of such materials include but are not limitedto various layered oxide(s), spinel(s), and olivines, to name a few.These include but are not limited to lithium cobalt oxide, LCO, lithiummanganese oxide, LMO, lithium nickel manganese cobalt oxide, NMC,lithium iron phosphate, LFP, various other lithium phosphates andfluorophosphates, various lithium metal silicates, and many others. Atthe same time, many conversion-type active materials offer highervolumetric Li capacities than intercalation compounds. In addition, someof them exhibit a specific Li insertion/extraction potential, which maybe advantageous for some applications. They are, however, mostlyincompatible with aqueous electrolyte solutions because they either (atleast partially) react with water or even (at least partially) dissolvein water (in some stage of charge or discharge). Examples ofconversion-type active materials include but are not limited toselenium, lithium selenide, sulfur, lithium sulfide, various metalfluorides (such as copper fluoride, nickel fluoride, iron fluoride,cobalt fluoride, and others), various metal chlorides, various metalbromides, various metal tellurides, various oxides, various nitrides,various phosphides, sulfides, various antimonides, and others. Someother intercalation-type electrodes may similarly exhibit undesirablereactions with aqueous electrolytes, but offer advantages for someapplications of aqueous Li-ion cells. Examples of such advantagesinclude a favorable Li insertion/extraction potential, high volumetricor gravimetric capacity, or a high Li insertion rate.

In order to overcome the incompatibility of some favorable activematerials with aqueous electrolytes, it may be advantageous in someapplications to enclose them in one or more Li-ion permeable, butsolvent impermeable protective shell(s).

FIG. 12 is a cross-section view of different example particle designsincorporating one or more Li-ion permeable, but solvent impermeableprotective shell(s). As shown, each of the example composite core-shellnanoparticles shown here is generally composed of a Li₂S core 1202 and aprotective shell 1204 that is permeable to Li ions, but not permeable toH₂O. In some particle designs, the core may further include carbonnanoparticles 1206 to enhance electrical conductivity. In some particledesigns, the core may further include a carbon matrix 1208 to enhanceelectrical conductivity. In some particle designs, the shell may beformed with a gradually changing composition 1210 as discussed above. Insome particle designs, the core may further include a porous scaffoldingmatrix 1212 to enhance electrical conductivity, as well as mechanicalstability.

In some applications (e.g., when the shells are electricallyconductive), it may be advantageous for such shells to be deposited onindividual particles prior to electrode assembling. In otherapplications (e.g., when the shells are electrically insulative or whenthe shells could be damaged during electrode processing), it may beadvantageous for such shells to be deposited after the electrodeassembling. In yet other applications, it may be advantageous to depositthe shells both times, before and additionally after electrodeassembling, for example to ensure the lack of water-permeable defects orweak points within shells.

The use of many conversion-type active materials (such as metalfluorides, sulfur, selenium, lithium sulfide, or lithium selenide, as afew examples) in aqueous Li-ion battery cells has been conventionallyimpractical because of their reactivity with (or solubility in) water.However, the above core-shell structure applied to such particles (whereshell(s) around the particles prevent water access to theconversion-type active material) may provide unique capabilities to suchLi-ion aqueous cells.

Examples of the electrically conductive, Li-ion permeable and waterimpermeable shell materials include but are not limited to graphitic,disordered, or amorphous carbon. In some cases, it may be advantageousto use various metals (such as copper, nickel, or iron, to name a few)or various metal alloys as conductive coatings. It may be important,however, to make sure that the deposited metals are further protectedagainst corrosion. It may be further important to make sure that themetal-coated electrodes are not exposed to potentials where undesirablephase transformation may take place. In some applications, it may beadvantageous to use conductive polymers (such as polyaniline, forexample) as a shell material.

Examples of electrically insulative shell materials include variousoxides (such as aluminum oxide, zirconium oxide, silicon oxide, orvarious mixed oxides), various fluorides, various sulfides, variousmixed ceramics, various polymers, various composites, and others. It maybe important to make sure that the electrode is not exposed to thepotential where undesirable phase transformation takes place. Forexample, titantium oxide should not be exposed to a potential belowaround 1.7 V vs. Li/Li+. It may also be important to make sure that theshell is compatible with the electrolyte employed (e.g., so that it doesnot dissolve in the electrolyte).

Similar to the protective shell(s) deposited for the purpose ofpreventing aqueous electrolyte decomposition, the shells deposited toprotect the active material from undesirable reactions with water maycontain multiple layers. These layers may similarly offer differentfunctions. For example, in addition to protecting the active materialfrom unfavorable interactions with aqueous electrolytes, these shellsmay provide one or more of the following functions: (i) enhanceelectrical connectivity between individual active particles; (ii)improve mechanical stability of the active particles; (iii) reducevolume changes within the active particles during cycling; and/or (iv)prevent aqueous electrolyte decomposition at extreme potentials (such asoxygen generation at a high potential of a cathode and hydrogengeneration at a low potential of an anode).

As discussed above, one layer may, for example, assist in electricallyconnecting active particles of the electrode. In this case, the layershould be electrically conductive. Examples of materials for such alayer include but are not limited to a conductive carbon coating or aconductive metal coating, which should be stable in the potential rangefor the electrode of interest. Nickel is an example of such a metalsuitable for some anodes. Aluminum is an example of such a metalsuitable for some cathodes. A layer can also be deposited in order toassist in forming uniform coating of a subsequent (e.g., second) layer.Examples of materials for such a layer include but are not limited tometal(s), metal alloy(s), metal oxide(s), metal fluoride(s), metalsulfide(s), various other ceramic coatings, polymer(s), andcomposite(s), to name a few. It may be important that this materialshould also be stable in the potential range for the electrode ofinterest and not undergo undesirable phase transformation reactions. Asdiscussed above, a layer can also be deposited in order to enhance themechanical properties of the overall coating or enhance the mechanicalstability of individual particles. Examples of materials for such alayer include but are not limited to carbon, metal(s), metal alloy(s),metal oxide(s), metal fluoride(s), various other ceramic coating(s), andcomposite(s), to name a few.

In some embodiments, active cathode particles comprising aconversion-type active material may be used in combination with anodeactive particles comprising an intercalation-type active material in aconstruction of aqueous Li-ion cells. In other applications, anintercalation-type active material can be used in the cathode and aconversion-type active material in the anode. In yet other applications,it may be advantageous to use conversion-type active materials for bothelectrodes or intercalation-type active materials for both electrodes.In still other applications, it may be advantageous to use both types ofLi storing materials (intercalation and conversion) in one electrode(for example, when a high capacity conversion-type active materialresiding in the core of an active particle is surrounded by a lowercapacity intercalation-type active material shell that stores Li ionsand simultaneously protects the core from unfavorable interactions withan aqueous electrolyte).

All layers with a shell should be permeable to ion transport in order toprovide energy storage capabilities to active particles.

In some applications, an overall thickness of the protective shell inthe range of about 5 nm to about 500 nm may be advantageous. Thinnershells may be prone to defects. Thicker coatings may impede the iontransport or contribute to a significant portion of the total mass orvolume, which is undesirable.

In some applications, it may be advantageous for the protective coatingto gradually change in composition. In this case, the internal stressesduring cycling may be reduced and delamination of the coating may beprevented.

In some applications, it may be advantageous for the conformalcoating(s) on the electrode surface to both (i) protect some of theactive material from reaction with the aqueous electrolyte and (ii)impede or prevent decomposition of the aqueous electrolyte at extremeelectrode potentials (that is, prevent oxygen generation on the cathodesurface or hydrogen generation on the anode surface). Methods describedabove may be used to produce pH-regulating layers on the surface of suchshells to enhance the aqueous stability range. Similarly, otherdescribed methods may be used to deposit layers of electricallyinsulative (yet Li-ion permeable) material on the surface of such shellsto further enhance the stability range of an aqueous electrolyte.

Various deposition techniques may be used for the conformal formation oflayers or complete shells for various implementations described above(such as preventing electrolyte decomposition or preventing variousundesirable reactions between the electrolyte and active material, toname a few). Examples include but are not limited to various vapordeposition techniques (such as chemical vapor deposition or CVD, atomiclayer deposition or ALD, plasma-enhanced CVD, and plasma enhanced ALD,to name a few), various wet chemistry deposition techniques (such aslayer-by-layer deposition, dip coating, solution precipitation, sol-gel,electroless deposition, and electro-deposition, to name a few) and otherknown techniques for the deposition of conformal layers on porouselectrode substrates or particles.

For example, for the formation of a nickel metal coating, a CVD methodmay be used that involves thermal decomposition of aNickel-biscyclopentadienyl (Nickelocene, Ni(C₅HS)₂, or NiCp₂) precursoror nickel-carbonyl (Ni(CO)₄) precursor at elevated temperatures (forexample, within a temperature range of about 180-250° C.). In someapplications (e.g., when a high degree of uniformity is required), itmay be advantageous to conduct CVD at reduced pressures (e.g., undervacuum). For the formation of a carbon coating (if the core is thermallystable), a suitable polymer layer may be deposited on the surface of theparticles (for example, by a solution precipitation method) andcarbonized by annealing at elevated temperature (e.g., above about 400°C.). Alternatively, a CVD method may be employed that involvesdecomposition of hydrocarbons (such as acetylene) in a gaseous phase atelevated temperature (e.g., above 400° C.). A combination of suchmethods can also be employed.

FIG. 13 provides an example of a high capacity aqueous Li-ion batterywith a pH-modified anode and cathode. Active cathode particles thatcomprise one of the common intercalation-type Li ion storing materials(such as lithium cobalt oxide, LCO, lithium manganese oxide, LMO, orlithium nickel manganese cobalt oxide, NMC) are used in this examplecathode embodiment of Li-ion aqueous cells. In some cases (for example,when active particles are designed to have small volume changes duringcycling and when their surface is protected from direct interactionswith water, as previously described) active anode particles may compriseconversion-type active material(s). In the current example, the anodecomprises environmentally-friendly low-cost sulfur (S)-based core-shellparticles that may offer over two times higher volumetric capacity thanthe graphite currently used in conventional organic Li-ion cells. Whilesome conventional designs have utilized S or Li₂S-comprising activematerial within a cathode (positive electrode) of a Li-ion or Li cellwith an organic or ionic liquid electrolyte, the use of ashell-protected S or Li₂S-comprising active material as an anodematerial with an aqueous electrolyte is unique.

Many high capacity active material exhibit significant volume changesduring insertion and extraction of Li ions. Such volume changes mayinduce defects in the functional conformal coatings previouslydescribed. Such defects may lead either to the undesirable reaction(s)of the aqueous electrolyte with active material or induce decompositionof the aqueous electrolyte, or both. It is therefore desirable foractive particles as a whole to have relatively small volume changesduring cycling, and to use such lower volume change particles in theconstruction of electrodes for aqueous Li-ion cells with enhanced cellvoltage.

Accordingly, in various embodiments, each of the active materialparticles may include internal pores configured to accommodate volumechanges in the active material during the storing and releasing of ions.When the active material is a high capacity material that changes volumeby more than about 10% during insertion and extraction of ions (e.g.,Li⁺, Na⁺, or Mg²⁺ ions), the internal porosity of the active particlescan be used to accommodate these volume changes so that charge/dischargecycles do not cause failure of the particle/protective layer interface,and do not induce formation of cracks in the protective layer(s). Theoverall porosity can be optimized to maximize the volumetric capacity,while avoiding the critical stresses that cause rapid composite failureor fatigue during battery cycling. In some applications, when arelatively brittle protective layer(s) is used or when the interfacebetween the electrode particles and the protective layer(s) isrelatively weak, then the presence of internal pores may prove to bebeneficial even when active material changes volume by less than 10%.

Such porous particles may be produced by a so-called “bottom-up”approach, where the particles are built from smaller building blocks.One example to produce such porous active particles is utilization of anemulsion route. For example, active material in the form ofnanoparticles can be dispersed in the suitable liquid. Binder (monomeror polymer) to keep the active nanoparticles together can be added tothe liquid as well. Another type of additive (conductive particles, forexample) can be dispersed jointly with the active materialnanoparticles. Then, the suspension of the active particles with thebinder may be emulsified in a second liquid immiscible with the first.The size of the porous particle may be controlled by the size ofemulsion droplets. The droplets of the emulsion may then be solidifiedby solvent evaporation or monomer polymerization, yielding porousparticles containing pores. In yet another example, porous particles maybe produce by a so-called “balling” method, according to which smaller(for example, nanosize) particles are agglomerated together using abinder, which can be removed at later stages or transformed into a solid(e.g., a solid carbon, by carbonization of organic binders). In someexamples, the particles can be further annealed in a controlledenvironment to induce sintering of individual nanoparticles. Anothergeneral route to produce such particles is a “top-down” approach wherepores are induced in solid particles. In one example, the porousparticles can be produced by first forming two or morecompound-comprising particles, where one compound is leached out bydissolution or vaporization. In yet another example, porous particlesmay be produced by partial etching of solid particles.

In some embodiments, it may be advantageous for the active particleswith internal porosity and volume-changing active material to be acomposite of (i) a conductive material that does not exhibit volumechanges (or exhibits very low volume changes) and (ii) volume-changingactive material. In some cases, it may be further advantageous for the“low volume change” material to provide a rigid scaffold with internalpores partially filled with a volume changing material. Thisarchitecture of the particles allows one to further minimize the volumechanges in such composite particles during cycling. Conductive carbon isan example of a material that may be used for such a scaffold.

FIG. 14 provides an example of different porous particle designscontaining a conversion-type active material (sulfur) that experiencesvolume changes upon Li insertion. As shown, the composite core-shellnanoparticles in this example are generally composed of a porous sulfurcore 1402 and a protective shell 1404 permeable to Li ions, but notpermeable to H₂O. In some designs, the core may further include a porousscaffolding matrix 1406 to enhance electrical conductivity, as well asmechanical stability. In some designs, the shell may be formed with agradually changing composition 1408 as discussed above.

In some embodiments, it may be advantageous for the thickness of thefeatures of the porous scaffold material to be small, e.g., in the rangeof about 0.3 to about 50 nm in size. Defective fragments of graphene(single or multi-layered with a thickness in the range from 0.3 to 50nm, for example), activated carbon, carbon nanotubes, graphite ribbons,carbon fibers, carbon black, dendritic carbon particles, and variousother carbon particles may serve as a scaffold material in someapplications.

In some embodiments, it may be advantageous for the porous compositeparticles to be a nano-composite.

In some embodiments, it may be advantageous for the pores within theactive particles to remain small, e.g., in the range of about 0.4 toabout 10 nm.

In some embodiments, it may be advantageous for the “nodes” of theactive material deposited within the scaffold to be small, e.g., in therange of about 0.5 to about 100 nm in size.

In some embodiments, it may be advantageous for the porous activematerial (or for the “nodes” of the active material deposited within thescaffold) to contain a secondary protective coating. In this case, ifthe conformal coating around the particles fails, this secondary coatingmay provide additional protection against undesirable side reactionswith the electrolyte.

In some embodiments, conformal shells around the porous compositeparticles may serve to prevent volume changes in the porous particles.In some applications, it may be advantageous for the shell to havegradually changing porosity or gradually changing composition, or both(for example, to minimize stresses occurring during battery cycling andimprove stability of the shell-core interface). It may further beadvantageous for the shell to gradually emerge from the porous core,again to minimize internal stresses and improve mechanical stability ofthe composite active particles.

The high rate capability of an aqueous electrolyte can reduce theoverall heating caused during use. In addition, high temperatureperformance will not cause significant irreversible degradation in anaqueous, pH-neutral Li-ion electrolyte. As such, battery structuresprovided herein require little or no cooling system. Because of theinherent safety of the cell, conventional packaging used to make batterymodules and packs can be reduced, as they are no longer needed to servethe same protective role. Instead, the battery module and packs can beused (e.g., in electric vehicle applications) to protect passengers andabsorb the energy of impact in the case of a severe crash (theelectrolyte is safe). This may further improve the system-levelperformance of the provided energy storage solution based on a pHneutral electrolyte.

FIG. 15 is a flow chart illustrating an example method of fabricating abattery electrode composition comprising active particles. As shown, themethod 1500 may comprise, for example, providing active materialparticles to store and release ions during battery operation (block1510) and electrically connecting the active particles with a currentcollector (block 1520). A conformal protective coating may then beformed on the electrode surface in such a way that the electrode remainsporous while all (or at least a significant portion) of its open poresurface area is covered with such a coating (block 1530).

For connecting the active particle together during the electrodefabrication, the method may utilize a mixing process for mixing theactive particles with a binder and an annealing process for annealing atan elevated temperature to cause solidification of the bonded particlesin a particular shape. In some embodiments, the surface of the activeparticles may allow sintering particles together at elevatedtemperatures and thus not require a binder. In some embodiments, thesurface coating of the active particles may deform during sintering orelectrode preparation (e.g., during annealing or during application of amechanical pressure) in such a way as to have a significantly smallercoating thickness in the areas where particles touch each other. Thismay be advantageous, for example, when the coating is electricallyisolative, because in the particle-to-particle contact points asignificantly thinner coating may provide, for example, paths forelectron transport (for example, via quantum mechanical tunneling).

As previously discussed, in some embodiments, the coating or shell hasgradually changing composition. This may be achieved, for example, bygradually changing the composition of the coating precursor.

In contrast to traditional Li-ion batteries, aqueous Li-ion cells can bemanufactured in a small, commodity, cylindrical form factor, which maybe advantageous for electric vehicle applications. For example, such amulti-cell battery can be designed to have a shape that fits the spaceavailable, rather than building the car around a large prismatic design.Small cylindrical cells using steel casings can be used to providetremendous rigidity to the module and pack, and in turn carry loadsnormally borne by the chassis. With traditional Li-ion cells, such anapproach would never be used, since damaging the cells in an accidentwould lead to nearly certain thermal runaway. This approach, however, ismade feasible by the aqueous Li-ion cells disclosed herein.

In some embodiments, it may be advantageous for the thicker electrodesof aqueous Li-ion batteries to contain pores (for example, poresperpendicular to the electrode surface) to provide channels for fasterLi-ion electrolyte diffusion through the electrode. The pore width mayrange, for example, from as little as about 20 nm to as much as about500,000 nm (0.5 mm). This structure of the porous electrode may beparticularly advantageous if the electrode thickness is in the range ofabout 0.2 mm to about 5 mm. In this case, having the “channel” poreswithin the electrode may significantly enhance the rate or powerperformance of such aqueous Li-ion batteries.

In some embodiments, it may be advantageous to embed a porous metal(e.g., a metal or conductive carbon foam or mesh) current collectorwithin the electrode. In this case, both mechanical properties of theelectrode and electrical conductivity of the electrode will be enhanced.It is noted, however, that it some embodiments (e.g., in cases when themetal current collector does not exhibit high over-potential for waterdecomposition), it may be advantageous to deposit a conformal protectivecoating on all of the open internal surface area of the electrode,including the current collector.

Compared to conventional Li-ion batteries, the dramatic cost reductionof the provided aqueous Li ion technology also comes from differentmanufacturing technology that could be enabled by the significantlyhigher ionic conductivity of aqueous Li-ion electrolytes. Becauseaqueous electrolytes offer higher conductivity than those based on thecarbonate solvents used in commercial Li-ion cells, the electrodes canbe made about 0.5-5 millimeters thick while maintaining acceptably highpower characteristics. This is because high electrical conductivity isrelatively easy to maintain and because relatively slow (e.g., less thanaround “2 C”) charging rate in graphite anode-based commercial Li-ioncells is limited by the low solid electrolyte interphase stability, highcharge-transfer resistance, and Li plating (due to low lithiatedgraphite potential). All these factors disappear or become greatlyreduced (charge transfer resistance) in aqueous Li-ion systems. As aresult, with thick electrodes, bulk (molding) rather than surface(coating) manufacturing methods may be used in some embodiments ofaqueous Li-ion batteries. In some applications, it may be advantageousto use a process that is akin to alkaline batteries rather thantraditional Li-ion cells.

FIG. 16 shows a comparison of two cell constructions, including aconventional Li-ion cell side by side an aqueous Li-ion cell asdescribed herein. A traditional Li-ion cell in a cylindrical 18×65 mmcase utilizes anywhere from 15 to 30 winds of a very thin electrode tooccupy that volume. In order to create the winding, great care is takento cast the active material onto thin copper and aluminum foils whichare then sliced into sections nearly three feet long, stacked with twoseparators, and wound with extreme precision to ensure all edges arealigned. Any misalignment or variation in the amount of active materialalong the three-foot foil can lead to electrical short circuits andthermal runaway. As a result, these processes require extremely highprecision and many additional quality control steps which result in arelatively high cost of assembly.

There are also technical limitations in this process. For example, theminimum thickness of Cu and Al that must be used to keep from tearingduring assembly is approximately 10 μm. Much of this foil, however, isunnecessary from an electrical conductivity standpoint, adding little tothe performance of the cell other than allowing for robust assembly. Thecopper and aluminum conductors in a cell make up 5 g of a 45 g cell orabout 11% of the total mass. The separator, while light, takes up 7% ofthe volume. The case adds 12-14% by volume and 10% by mass. Much of thisis essentially dead weight, as well as dead volume and unnecessary cost,which are compared below.

This construction methodology leaves only 60-65% of volume available forthe functional active electrodes in the cell. The reason for thiscomplexity and inefficiency stems directly from the need to keepelectrode thicknesses at or below 100 μm to allow sufficient ionicconductivity in the electrode during operation. The need for electricvehicles, for example, to operate at low temperatures exaggerates theselimitations even further, as the ionic conductivity of the commercialorganic electrolytes often drops tenfold when operating at −20° C.Finally, due to the high sensitivity of cell performance to moistureresidues, extensive drying and expensive glovebox-operated electrolytefilling/sealing protocols must be employed.

In contrast, assembly for the provided aqueous Li ion technology isdramatically simpler. As in alkaline cells, a cylindrical pellet ofanode material may be prepared, typically about 0.5-8 mm thick dependingon the diameter of the battery, and inserted into the casing from theopen top end. The pellet is electrically conducting and free standing,and makes contact with the casing, which serves as a current collectorand negative terminal for the cell. Next a cylindrical separator isinserted, after which a cylindrical cathode pellet, followed by theaddition of the electrolyte, the top cap, and the positive electrode pin(which occupies the same space and doubles up functionally for thetraditional central vent tube). Once firmly pressed, the cell is crimpedin a manner similar to conventional cells.

Unlike conventional Li-ion cells, however, the entire process can takeplace in a humid environment and does not require the construction ofexpensive dry rooms. The simple construction is not only cheaper andfaster to manufacture, but carries additional safety benefits andenhanced process robustness. In traditional Li-ion construction, theseparator spans nearly three feet, and two layers are required for thewinding. As a result, engineers have pushed the separator to be as thinas possible to minimize its inactive volume—anywhere from 16-25 μm intypical cells. This, however, reduces the safety of the cell, as thethinner separators are more susceptible to internal short circuits dueto defects, particulate contaminants, and dendrites. A penetrationthrough the separator during charging is a common cause of suddenthermal runaway in Li-ion systems. To combat the problem, automotivecells use thicker separators—typically, 25 μm and thicker—but thisreduces the energy density of their cells and increases the $/kWh cellcosts. In the construction provided herein, however, the separatorlength is less than about 1/20^(th) of that in a conventional cell, andcan therefore be made thicker to improve safety and eliminate unwantedinternal short circuits with minimal impact on cost or energy density.

In contrast to traditional alkaline cells, in some embodiments, it maybe advantageous to use more than one positive or more than one negativeelectrode in the construction of the aqueous Li-ion cells. In this case,the thickness of each electrode may be kept relatively small (forexample, about 0.2-1 mm), while the overall power performance may behigh, allowing fast charging (within an hour or faster) in cells with arelatively large diameter of more than 10 mm.

In some embodiments, it may be advantageous to produce planar cells,instead of cylindrical cells. In this case, cells may be packed togethermore efficiently, providing less “free volume” space between individualcells.

FIG. 17 shows select performance characteristics of the two cellconstructions, including a conventional Li-ion cell side by side anaqueous Li-ion cell as described herein. Deconstruction of amass-produced, 2.9 Ah, 3.6 V traditional Li-ion cell showed the anodeand cathode capacity with a volumetric capacity to be 400 and 600mAh/cc, respectively. Because certain example embodiments may utilize asimilar, traditional cathode with a surface modification technique, theymay also reach 600 mAh/cc in well-designed cells. Capacity of pure Li₂Sis 1,931 mAh/cc. Conservatively assuming that 48% of the volume will beoccupied by the non-active components and pores, it can be estimatedthat the protected S-based anode capacity may approach 1,000 mAh/cc forthis example of an aqueous Li-ion cell. Since a different manufacturingtechnology can be employed for the fabrication of aqueous Li-ion cells,the volume occupied by the separator may be reduced, and the Al and Cufoils may be eliminated. As a result, for an 18650-volume-equivalentaqueous Li-ion cell with such 1000 mAh/cc anode and 600 mAh/cc cathode,it may be estimated that a 5.3 Ah capacity may be achieved, along withan average voltage of, for example, 1.9 V, and an energy density of 610Wh/L (200 Wh/kg). This is around 90% of traditional high energy Li-ioncells, but at substantially lower cost.

The forgoing description is provided to enable any person skilled in theart to make or use embodiments of the present invention. It will beappreciated, however, that the present invention is not limited to theparticular formulations, process steps, and materials disclosed herein,as various modifications to these embodiments will be readily apparentto those skilled in the art. That is, the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention.

What is claimed is:
 1. A metal-ion battery composition, comprising: anode and cathode electrodes, wherein at least one of the electrodes comprises a plurality of active material particles provided to store and release ions during battery operation; and an electrolyte ionically coupling the anode and the cathode, wherein the electrolyte comprises an aqueous metal-ion electrolyte ionically interconnecting the active material particles, wherein the plurality of active material particles comprises a conformal, metal-ion permeable coating at the interface between the active material particles and the aqueous metal-ion electrolyte, whereby the conformal, metal-ion permeable coating impedes water decomposition at the at least one of the electrodes. 